The turbine sections of gas turbine engines contain stationary components that serve as an outer ring and sealing surface to reduce interstage gas leakage past the turbine blade tips. In industrial gas turbines (IGT's) these stationary components are termed “ring segments” and in aero engines they are called “blade outer air seals” or BOAS. Both aero and IGT components in the hot working gas path are coated with an oxidation resistant metallic layer called a bondcoat, and one or more layers of thermally insulating ceramic coatings. In addition, the outermost ceramic layer(s) are purposely sprayed with a low density (porous) to enable them to perform as clearance control coatings known as abradable coatings. When the metal turbine blades rub against the porous abradable coating, the blades will cut a swath through the coating without damaging the blade, thus allowing tighter clearance between the blade tips and the ring segments to minimize hot gas leakage between turbine stages. This has a beneficial effect on both power output and fuel efficiency, so abradable coatings are very important to engine designers.
Abradable coatings are typically sprayed much thicker than other types of thermal barrier coatings because their thickness is required to allow the blade tips to cut into the coating the desired amount. As the coating thickness increases, the thermal strains in the coating during startup and shutdown of the engine also increase. This can result in spalling of the abradable coating, and loss of both clearance control (engine power and efficiency) and thermal protection (metal ring segment service life).